A superlattice is a semiconductor structure in which periodic variations in the bottom limit of the conduction band and in the top limit of the valence band appear along one of the axes of the semiconductor (its growth axis).
In practice, a superlattice is defined by a periodic stack of two different kinds of layer on a substrate, the different kinds of layer being constituted by two different compounds or else being doped in different manners.
As a result potential wells appear both in the conduction band (in "dents" for electrons) and in the valence band (in "bumps" for holes).
As a monocrystal, a superlattice is thus a material which is substantially isotropic in the two dimensions of each of its layers. However, it is strongly anisotropic in the third dimension running parallel to its growth axis.
In this respect, numerous situations may be distinguished, depending on the characteristics of the superlattice, and in particular on the thicknesses and the compositions of the layers, which determine whether the electron (or the hole) wave function confers a significant probability on the electrons (or the holes) of reaching an adjacent quantum (potential) well by means of the tunnel effect:
if the probability is substantially zero, then there is a succession of decoupled quantum wells; it is doubtful whether the term "superlattice" is applicable to such a structure;
if the probability of tunneling is significant, then there is a genuine "superlattice"; and
if the probability of tunneling is very high, then the structure tends to have the same behavior along its growth axis as it does along the two directions perpendicular thereto, and again it is doubtful whether the term "superlattice" is applicable to such a structure.
At present, there is intense research activity concerning electro-optical modulators as components for use in optical fiber telecommunications.
Most of the recently published results make use of the quadratic Stark effect presented by the fundamental transition of an exciton (an electron-hole pair) into a GaAs-Al.sub.x Ga.sub.1-x As quantum well (see the article by D. A. B. Miller, J. S. Weiner, and D. S. Chemla, entitled "Electric Field Dependence of Linear Optical Properties in Quantum Well Structures: Wave Guide Electro Absorption and Sum Rules," in IEEE Journal of Quantum Electronics QE22, September 1986, page 1816; and the article by D. A. B. Miller, entitled "Novel Optical Modulators and Bistable Devices Using the Self-Electro-Optic Effect in Semiconductor Quantum Wells," in Surface Science 174, 221, 1386).
In outline the effect is as follows: when an electric field is applied perpendicularly to the plane of the layers, the optical absorption threshold is shifted towards lower energies by an amount (typically a few millielectron-volts) which is proportional to the square of the applied voltage. It is thus possible to use an electrical voltage to vary the transmission of light having a wavelength close to the absorption threshold.
Using this principle and various non-linear properties, it is possible to design a variety of integrated opto-electronic components for optical fiber telecommunications.
Two technical aspects should be observed:
the use of exciton resonance limits this type of component, in practice, to the 0.8 micron range (applicable to GaAs material), since the exciton effects are much greater in a material having the wide forbidden band, whereas the most advantageous "windows" appear to be at 1.3 microns, 1.5 microns, and 4 microns; and
it appears that the sign of the shift (towards low energies or towards the red end of the spectrum) implies that these components are always "normally transparent" which is a drawback from the dissipation point of view.
Although the Stark effect does not, a priori require a superlattice (in the general meaning of a periodic structure), it will be understood that it applies to a superlattice, essentially in the first of the above-mentioned categories, i.e. a succession of non-coupled quantum wells.
The present invention relates to a different way of using a superlattice as an electro-optical modulator.
The main aim of the invention is to provide a structure which does not operate by using exciton resonance, and which is therefore not limited to the range of optical wavelengths situated around 0.8 micrometers.
The invention also seeks to provide a structure in which the absorption threshold is shifted towards high energies (and not towards low energies) in response to the application of a controlling electric field.